Rivers died Sept. 4 after what was to be a routine procedure

Joan Rivers had an unplanned throat biopsy that caused her to stop breathing and ultimately led to her death, The New York Daily Newsis reporting, citing an anonymous medical source.

Rivers had originally gone to Yorkville Endoscopy to undergo a routine vocal cord surgery. The Daily News source says a doctor, who was part of Rivers' entourage, volunteered to perform the biopsy after another doctor spotted something on Rivers' vocal cords.

According to the source, Rivers' vocal cords seized during the biopsy and she stopped breathing. The source adds such procedures are generally only performed at hospitals, and Rivers might have recovered had she been in a hospital when it occurred.